1. Field of the Invention
The present invention relates to a semiconductor device tester using electron beam and, particularly, to a semiconductor device tester in which current flowing through a semiconductor device irradiated with electron beam is measured.
2. Description of Related Art
In a semiconductor device such as memory, contact-holes or via-holes are usually provided for electrically connecting active elements formed in a lower portion thereof to a wiring layer formed in an upper portion thereof. The contact-holes are formed by etching an insulating film such as an oxide film from a surface thereof down to an underlying substrate by reactive ion etching. In order to optimize etching condition, it is necessary to detect an outer and inner configurations of a contact-hole or a state of a bottom of the contact-hole.
Since the diameter of contact-hole is in the order of microns or less, visible light can not illuminate the bottom of the contact-hole, so that it is difficult, to detect the state of the contact-hole optically. Therefore, SEM (Scanning Electron Microscope) suitable for analysis of a fine structure has been mainly used as a tester. In the SEM, a contact-hole region is irradiated with electron beam, which is accelerated to several tens keV and collimated to several nanometers, and secondary electron produced in the irradiated region is detected by a secondary electron detector, on which an image of the contact hole is formed. A specimen irradiated with the electron beam generates secondary electrons, an amount of which corresponds to constituting atoms thereof. However, the secondary electron detector in the SEM is usually arranged in a specific direction, so that a whole of produced secondary electrons are not always detected. If the specimen includes irregularity in its structure, there is a case where secondary electron is not detected depending upon portions of the specimen, resulting in that contrast is produced in an image of the specimen under test, which is formed of a single substance. This is the feature of the SEM.
On the other hand, an electrical contact is realized through a contact-hole or a through-hole. Therefore, not only a configuration of an opening portion of the contact-hole but also a configuration and a surface condition of the bottom portion of the contact-hole are very important. In an etching for forming contact-holes each having aspect ratio exceeding 10 in concomitant with the recent increase of integration density and the number of layers of a semiconductor device, there may be a case where inner diameters of the contact-holes become different from diameters of opening portions of the contact-holes depending upon process condition even when the sizes of the opening portions are the same as a designed size. Since such variation of the inner size of the contact-hole substantially affects characteristics of a semiconductor device, it is necessary for persons in charge of a process to control the process such that all contact-holes have identical sizes. Further, since such size variation of contact-holes must not exist in practical products, the products have to be tested too. Therefore, a technique capable of non-destructively detecting both the inner size of the contact-holes and such size variation of the contact-holes is very important.
FIGS. 4(a) and 4(b) illustrate a test method using an SEM for testing a contact-hole 43 having a circular cross section and a result of the test thereof, respectively, and FIGS. 5(a) and 5(b) illustrate a test method using an SEM for testing a tapered contact-hole and a test result thereof, respectively. In the test using the SEM, the specimen under test is scanned by electron beam 31 and secondary electron 32 produced in the specimen is detected by a secondary electron detector 33.
It is assumed that the circular contact-hole 43 is formed through an insulating film 41 such as an oxide film formed on an underlying substrate 42 by etching from an opening portion thereof in a vertical direction such that the contact-hole 43 has an inner diameter substantially equal to a diameter of the opening portion, as shown in FIG. 4(a). In such case, energy of secondary electron hardly reaches the detector 33 unless there is a space large enough to gather a sufficient amount of energy since the energy of secondary electron is small. Therefore, a measured amount of secondary electron becomes as shown in FIG. 4(b). That is, an image of secondary electron obtained becomes suddenly darkened correspondingly to the opening portion of the contact-hole 43. By this phenomenon, an existence of a contact-hole is detected.
On the other hand, it is assumed that a contact-hole 44 has a tapered configuration whose diameter is reduced with depth as shown in FIG. 5(a). In such case, secondary electron from the tapered portion of the contact-hole may be observed depending upon a position of a secondary electron detector. However, since the aspect ratio of the contact-hole 44 is large, secondary electron emitted from an inner wall of the contact-hole can not observed practically. Therefore, the configuration of the contact-hole 44 and an information of a bottom thereof are not always reflected to a secondary electron image.
In the tapered contact-hole such as shown in FIG. 5(a), the inner diameter thereof is reduced with increase of a depth thereof and there may be a case where a contact resistance of the contact-hole is increased, resulting in a defective contact-hole even if the diameter of the opening portion thereof is acceptable. In the SEM test, however, a detected image becomes dark sharply at the opening portion of the contact-hole and an information of a bottom thereof is not reflected to the image regardless of whether the configuration of the contact-hole is circular or tapered. Thus, it is impossible to distinguish these contact-holes by the usual SEM.
In order to test an interior or a bottom of a contact-hole, a method of observing a cross section of the contact-hole of a specimen obtained by vertically cutting the specimen along a center axis of the contact-hole has been employed. This method requires a high level technique for precisely cutting the specimen to two pieces at the center axis of the contact-hole. Therefore, in view of the diameter of the current contact-hole in the order of several thousands Å, it is practically impossible to cut the specimen along the center axis of the contact-hole with precision of 10% which is necessary to determine the quality of a product. Further, this method is a destructive test and requires considerable labor and time, in addition to the impossibility of directly observation of the product.
In order to solve such problems, JP H10-281746A discloses a technique in which current produced by electron beam, which is passed through a contact-hole and arrived at a substrate, is detected to detect a position and size of a bottom of the contact-hole. Further, JP H4-62857A discloses a technique in which a secondary electron image is obtained by irradiating a contact-hole with not electron beam but ion beam and measuring a current flowing through a substrate due to the ion beam irradiation.
As another prior art, JP H11-026343A discloses a technique in which a pattern for measuring a positional deviation of a mask is formed and an amount of positional deviation of the mask is obtained on the basis of a substrate current produced when electron beam irradiation is performed. Further, JP P2000-174077A discloses a technique in which an area containing a plurality of contact-holes is irradiated with electron beam and a ratio of normal contact-holes in that area is tested on the basis of current values produced by electron beam passed through the contact-holes.
Further, it is possible to know a film thickness by measuring a substrate current. For example, JP P62-19707A discloses a technique in which a relation between a waveform of a substrate current, acceleration voltage of electron beam and a film thickness, when a pulsed electron beam irradiation is performed, is preliminarily obtained and a fir thickness is obtained from a current waveform measured by using electron beam accelerated with a certain acceleration voltage. Further, JP P2000-124276A discloses a technique in which a current, which is not a variation of current with time but a current value, produced by irradiating a test sample with electron beam and passed through the test sample to a backside surface thereof is measured. In a technique disclosed in JP 2000-180143A, a current flowing through a thin film to a substrate, is measured and a film thickness is obtained by comparing the measured current with a current value obtained for a standard sample and JP P2000-164715A discloses a standard sample suitable for use in the technique disclosed in JP P2000-180143A.